The present invention relates to the control of stepper motors, and more particularly, to a stepper motor control circuit with relatively low power dissipation which produces sufficient motor torque to enable acceleration of an applied mechanical load without stalling of the motor.
In a stepper motor each revolution of the motor shaft is made in a series of discrete identical steps. The design of the motor usually provides for clockwise and/or counter clockwise rotation. A stepper motor is ideally suited for many positional and control applications. It permits use of digital signals to control mechanical motion or position. In addition, the high holding torque associated with each step permits a stepping motor to replace devices such as brakes and clutches, with a gain in system reliability.
Typically a stepper motor will stall if its armature gets more than two steps out of phase due to an applied mechanical load. It is therefore important to drive the phase windings of the stepper motor with sufficient current to provide the torque necessary to accelerate the applied mechanical load. This requires special circuitry since the stepper motor acts like an inductor.
In modern applications of the stepper motor, a control circuit decodes a digital input signal to provide a plurality of phase signals in a predetermined relationship for driving the stepper motor when amplified and applied to corresponding ones of its phase windings. In the control circuit a switching regulator or an L/R drive is typically utilized to overcome the motor's inductive time constant to enable the phase windings to be driven with enough current to permit an applied mechanical load to be accelerated. L/R drives typically dissipate large amounts of power and thus stepper motor control circuits which incorporate them are inefficient and generate substantial heat. Switching regulators offer efficient power control but do not have fast enough response time to provide the acceleration required in some applications. Known stepper motor control circuits have also utilized resistors and diodes to prevent back emf signals in the phase windings of the motor from damaging driver transistors in the circuit. The manner in which these components have been connected has further increased the power dissipation and heat generation of such control circuits.